1. Field of the Invention
The present invention is directed to a cardiac lead suitable for use in pacemakers, cardioverters, defibrillators, and the like, as well as to a method and circuit for using such a lead to detect cardiac rhythm abnormalities, such as fibrillation and tachycardia.
2. Description of the Prior Art
A cardiac lead typically has a proximal end with a connector adapted for electrical and mechanical connection to a cardiac-assist device, such as a pacemaker, cardioverter or defibrillator, and an opposite distal end, at which one or more electrodes is/are located. Between the distal end and the proximal end, the lead has a flexible insulating sheath or jacket, containing one or more conductors, depending on the number of electrodes.
The electrodes are exposed conductive surfaces at the distal end of the lead. Conventional electrode configurations include a unipolar configuration and a bipolar configuration. In a unipolar configuration, there is only one electrode at the distal end, typically a hemisphere covering the distal tip. Typically the housing, or a portion thereof, of the cardiac assist device is used as the indifferent or return electrode. A bipolar lead has two electrode surfaces, separated from each other by a slight spacing. Typically one of these electrodes is formed as a hemispherical electrode at the distal tip of the lead, and the other is a ring electrode, which annularly surrounds the sheath, located a short distance behind the tip electrode.
In most modern cardiac assist devices, the electrode lead is not only used to deliver an appropriate cardiac assist regimen in the form of electrical pulses, but also is used to detect cardiac activity. The detection of cardiac activity can serve many purposes, such as for use in determining whether adjustments to the cardiac assist regimen are necessary, as well as for identifying cardiac rhythm abnormalities which may require immediate preventative action, such as the occurrence of tachycardia or fibrillation. Particularly in the case of a cardioverter or a defibrillator, which is normally passive unless and until tachycardia or fibrillation is detected, it is important not only to reliably detect tachycardia or fibrillation when they occur, but also it is important not to misidentify a non-emergency cardiac rhythm abnormality as tachycardia or fibrillation, since administering the emergency regimen to a healthy heart can possibly create an emergency situation where none exists. Moreover, at least in the case of a defibrillator, unnecessary triggering of the extremely strong defibrillation energy will cause considerable discomfort to the patient.
An electrode lead for a cardiac pacemaker is disclosed in U.S. Pat. No. 5,306,292 which has a distal tip with a number of closely spaced electrodes thereon, with the remainder of the hemispherical surface of the distal tip of the electrode being non-conducting. Circuitry in the pacemaker housing, connected to the respective electrodes via the electrode lead cable, allows the total conductive area and geometry of the distal tip of the electrode lead to be selectively varied, by activating the electrodes in different combinations. For example, the combination of electrodes (i.e. conductive surfaces) at the electrode tip which provides the lowest stimulation threshold can be determined by an autocapture unit, so that energy consumption can be reduced.
Many algorithms are known for analyzing the detected signal wave forms obtained with unipolar and bipolar leads. A prerequisite to the proper functioning of most of these algorithms is that the signal which enters into the algorithm be relatively noise-free. The detected signal, in its raw form, can be corrupted by noise produced by electromagnetic interference in the patient's environment, as well as by muscle activity. Such noise may mimic a fibrillation pattern, for example, particularly in the case of a unipolar lead, but also to a certain extent with a bipolar lead.
Conventional noise-removing techniques involve filtering and other types of signal editing procedures.
After making the incoming signal reasonably noise-free, conventional detection algorithms analyze the signal by undertaking one or more threshold comparisons and/or by analyzing the rate of occurrence of a particular characteristic of the signal (i.e., maxima, minima, zero crossings, etc.) over a given period of time. Comparison of the signal waveform to stored signal templates, respectively representing previously-obtained abnormal signals, is also a known technique. In this manner, a determination is made as to whether the incoming signal represents normal sinus rhythm, a PVC, tachycardia, atrial fibrillation, ventricular fibrillation, etc.